Fluoropolymers are generally manufactured under emulsion or suspension conditions in water. A dispersion of the monomers in water is prepared in a stirred reactor using a surfactant and then a chain-transfer agent and an initiator are added. The polymerization is carried out and then the fluoropolymer (it is in the form of solid particles) and the water are separated by any means.
Depending on the nature of the surfactant and its proportions, the process is an “emulsion” process, a “suspension” process or any other process derived from the emulsion or from the suspension (microsuspension, miniemulsion, and the like). After the end of the polymerization, the fluoropolymer is separated from the water and possible residues of the reactants used.
In the case of suspension-type processes, the fluoropolymer exists in the form of a granular dispersion, the mean size of the grains of which makes it possible to directly filter and wash, for example by passing pure water into the filtration system.
In the case of emulsion-type processes, the polymer exists in the form of a latex composed of very fine particles, the mean diameter of which is generally less than 1 micron. This latex can be coagulated and optionally concentrated by removing a portion of the water, for example by centrifuging. In the coagulated state, it is also possible to obtain an aerated cream, less dense than water, which can be washed with deionized water according to techniques already described in the prior art (U.S. Pat. No. 4,218,517 and EP 0 460 284). The washed cream can then be dried by bringing it into contact with a hot gas in an atomizer and the fluoropolymer is collected as a powder.
In all these processes, water is obtained as by-product, but a gaseous effluent, which can comprise residual matter from the manufacturing operation, such as fluorosurfactants, possibly traces of solvent and various suspended solid impurities, is also obtained as by-product. The invention relates to the recovery of these surfactants. This recovery is desirable first because these surfactants might be dangerous to the environment and also because of their high cost. Among fluorosurfactants, the invention relates more particularly to ammonium perfluorooctanoate.
U.S. Pat. No. 3,882,153 discloses the passage of aqueous solutions of fluorosurfactants over ion-exchange resins. The fluorosurfactants are retained on the resin. U.S. Pat. No. 4,282,162 discloses a process similar to the above but discloses in addition the elution by an inorganic acid (HCl) and a solvent (methanol) to regenerate the resin and recover the surfactant. U.S. Pat. No. 6,613,941 discloses a process for isolating fluorosurfactants present in wastewater. The very fine solids and/or the fractions which can be converted to solids are first removed from the wastewater, preferably by precipitation, the wastewater is subsequently brought into contact with an ion-exchange resin and the fluorosurfactants are eluted from the latter. U.S. Pat. No. 6,518,442 discloses a similar process except that, in the preliminary stage, the very fine solids are not removed but are stabilized with another non-ionic surfactant, which surfactant is therefore not retained on the resin, which would make it possible to avoid blinding of the resin column by the solid particles. U.S. Pat. No. 6,436,244 discloses a process for eluting fluorosurfactants after their adsorption on a resin.
The publication by D. Prescher et al, Umweltverhalten von fluortensiden [Environmental behaviour of fluorosurfactants], in Acta hydrochim. hydrobiol., 14(3) (1986), pages 293–304, describes the removal of surfactants present in wastewater by bringing it into contact with active charcoal or ion-exchange resins. For example, the operation is carried out batchwise by placing the wastewater in a vessel equipped with a stirrer, by then adding powdered active charcoal thereto and by stirring for 30 minutes. A continuous process by passing the wastewater over granular active charcoal or over ion-exchange resins is also described. Surfactants of sodium perfluoroalkanoate type are found to adsorb well on powdered active charcoal and 10 to 30 times less well (that is to say that 10 to 30 times less surfactant is adsorbed per gram of charcoal) on granular charcoal. No elution is described and, a fortiori, no regeneration of the charcoal or of the surfactant for the purpose of their reuse is described.
It has now been found that it is possible to completely remove surfactants of the ammonium perfluoroalkanoate type present in wastewaters or gaseous effluents by passing these waters or these effluents over granular active charcoal and then that they can be eluted and regenerated, along with the active charcoal.